1. Field of the Invention
The invention relates to the field of controlling sludge flows pumped with a pump and to valves used for throttling such sludge flows.
2. Description of the Prior Art
The variation of rate of flow of a fluid through a pump can be necessary or desirable for a number of reasons, for example: the data upon which the design of an installation is based may not be in accordance with the actual process conditions encountered in operation; an installation may not be operated at full capacity; the required rate of discharge for production may require variation in the flow rate; and the pump may be set up for changing conditions, as for example in a pilot or experimental plant. The pump must then be adapted to fit the changed conditions and the flow rate must be controlled. These facts are well recognized in the art. See, for example, G. Leuschner, Kleines Pumpenhandbuch fuer Chemie und Technik, pp. 77 - 79 (1967).
Pump control has usually taken place either by changing the number of revolutions of the pump or by throttling the flow on the pressure side of the pump by means of a valve. When sludge flows are concerned, pumps are also often used without any control.
Regulating the number of revolutions of a pump without steps is absolutely the best control method, but a very expensive one. This high cost is noted at p. 79, line 22 of the aforementioned Leuschner handbook. Controlling the flow rate of a pump by throttling the pressure side is a method almost inapplicable to sludge pumps for the following reasons:
No control valves on the market endure high sludge flow rates. The life of the tube of a tube valve, which is often used, may be only 3-4 days. The pressure of the box water should be 0.5 kp/cm.sup.2 above the pressure on the pressure side. The great pressure impulses caused by a control valve placed on the pressure side prevent a continuous flow of the box water and the sludge may flow over through the box against the bearing bracket, thereby causing the box structure to wear out prematurely and shortening the life of the bearings.